Recently, interest in energy storing technologies continues to increase. As application areas expand to energies for mobile phones, camcorders and notebook PCs and further to electric vehicles, more systematic efforts are made for the researches and developments of electrochemical devices. In this aspect, electrochemical element is the area gathering the most attention, and most of all, developing a secondary battery capable of charging and discharging is becoming the focus of attention, and recently, in developing such batteries, research and development is under way to design new electrodes and batteries in order to improve capacity density and specific energy.
Among the currently-available secondary batteries, lithium secondary battery developed in the early 1990s is coming into spotlight because of advantages such as higher operating voltage and superior energy density compared to conventional batteries such as Ni-MH, Ni—Cd, lead-sulfate batteries, and the like that use an aqueous solution electrolyte.
As technological development and demand for mobile devices increase, demand for secondary batteries as an energy source is rapidly increasing, and recently, use of secondary batteries as a driving source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) is being realized. Accordingly, numerous researches are being conducted on secondary batteries that could respond to various demands, and especially, there is high demand for lithium secondary batteries having high energy density, high discharging voltage and output stability.
A lithium secondary battery consists of a cathode, an anode, an electrolyte and a separator. Particularly, the separator is interposed between the cathode and the anode so as to electrically insulate the cathode and the anode from each other, and it is required to have the characteristic that increases the permeability of lithium ions based on high porosity so as to increase the ion conductivity. The separator is generally made of a polyolefin-based material or a cellulose. The polyolefin-based polymeric material is advantageous in forming pores, has excellent chemical resistance, mechanical property and thermal characteristic, but also has extreme thermal shrinkage at high temperatures, and physical vulnerability. The cellulose can be used to prepare a separator having high physical strength, but is problematic in that it cannot serve the function of breaking current in an abnormal operating environment.